In recent years, in view of global environment conservation, to improve fuel efficiency of automobiles and to improve collision safety of automobiles, development has been aggressively carried out to reduce the wall thickness of automobiles by increasing the strength of materials therefor so as to reduce the weight of the automobile body itself and to increase the strength thereof, and hence high tensile strength steel sheets have been increasingly used in automobile applications. Since the high tensile strength steel is inferior in formability to soft steel, a high tensile strength steel sheet having improved formability has been developed through various structure controls. Furthermore, since improvement in corrosion resistance has been strongly requested for recent automobiles, a high tensile steel sheet processed by hot-dip galvanizing has been developed.
As a conventional technique, for example, Japanese Unexamined Patent Application Publication No. 2005-256089 has proposed a high-strength hot-dip plated steel sheet having excellent hole expansion properties, and Japanese Unexamined Patent Application Publication Nos. 2005-200690, 2005-200694 and 2006-299344 have proposed a high-strength hot-dip plated steel sheet having an excellent anti-powdering property and ductility.
However, to ensure a high strength of TS 590 Mpa or more, according to the techniques described above, it is necessary to add 0.25% or more of Al, and hence there have been problems of alloying cost, degradation in casting properties caused by Al addition, and the like. In addition, in particular, according to Japanese Unexamined Patent Application Publication Nos. 2005-200690, 2005-200694 and 2006-299344, since retained austenite is contained, although the steel sheet has high elongation properties, cracking may occur in secondary machining, and/or shape fixability of formed parts may be inferior to that of ferrite/martensite steel in some cases.
In consideration of the problems described above, it could be helpful to obtain a high-strength galvanized steel sheet, which has excellent formability equivalent or superior to that of a conventional high-strength galvanized steel sheet, which is manufactured at a cost equivalent to a conventional high-strength galvanized steel sheet, and which has manufacturing properties equivalent to those thereof, and a method for manufacturing the above steel sheet.